1. Field of the Invention
This invention relates generally to the field of holography and more particularly to a novel holographic recording unit and to a compact mobile holographic instrument embodying the recording unit for evaluating the structural integrity of load-bearing structures utilizing holographic interferometry.
2. Prior Art
As will become readily apparent from the ensuing description, the recording unit of the invention may be utilized for general purpose holographic recording and the holographic instrument may be utilized to evaluate the structural integrity of virtually any structure. However, the recording unit and holographic instrument are particularly adapted for evaluating the integrity of aircraft structures and other similar redundant load-bearing structures, that is load-bearing structures having multiple load paths. For this reason, the invention will be described in connection with this particular use. In view of the above noted broader utility of the instrument, it will be understood that this described application involving aircraft inspection is purely illustrative and not limiting in nature.
An aircraft wing is a highly redundant load-bearing structure having multiple internal load-bearing members providing multiple load paths through the structure. The wing structure is designed to sustain loads substantially in excess of those which are encountered in normal aircraft service. Over a period of time, however, a wing structure is prone to loss of its structural integrity, that is weakening of its load-bearing members due to fatigue damage, stress corrosion cracking, and other causes. Fatigue damage, of course, involves cracking of the wing load-bearing members, loosening of joints and rivets, and other weakening of the wing structure caused by the frequent load reversals which occur in the structure during flight, landing, and takeoff. Stress corrosion cracking occurs in aircraft which operate in an ocean environment and is caused by the corrosive action of salt water. In order to assure continued safe aircraft operation, therefore, it is necessary to periodically evaluate the structural integrity of aircraft wings, as well as other parts of the aircraft, of course.
A variety of inspection and testing techniques have been devised to evaluate the structural integrity of aircraft wings and other aircraft parts. One common inspection technique, for example, involves installing accelerometers on selected structural members for counting stress reversals experienced by the members. From these counts and a statistical model bases on the behavior of the particular aircraft structure of interest and statistical considerations regarding the distribution and size of defects, fatigue damage may be predicted. At appropriate times, the wing structure may be disassembled and subjected to actual fatigue inspection using x-rays or other nondestructive inspection techniques and/or fatigue damage tests. This method of evaluating structural integrity, however, is extremely costly and time consuming. The same applies to the current methods of inspecting aircraft wings and other structures for stress corrosion cracks, which methods require stripping all paint from the surfaces to be inspected, inspection of the surfaces by ultrasonic or other inspection techniques, and repainting of the surfaces. Accordingly, there is a need for an improved nondestructive inspection technique for evaluating the structural integrity of load-bearing structures, particularly highly redundant load-bearing structures, such as aircraft wings and other aircraft structures and parts.
The earlier mentioned copending application Ser. No. 456,998, provides such an improved inspection or structural integrity verification technique, and apparatus for its practice, involving holographic interferometry. This improved inspection technique is based on the fact that any loss of structural integrity, that is weakening, of a load-bearing structure due to fatigue damage, stress corrosion cracking, or other causes reduces the effective stiffness of the structure. Such reduction in stiffness, in turn, changes the deformations which the structure will experience in response to any given loading or stressing of the structure. The improved inspection technique utilizes holographic interferometry to detect such distortion changes and thereby changes in the structural integrity of the test structure.
According to the improved inspection technique, a load-bearing structure is periodically inspected by establishing in the structure two successive predetermined stress conditions of differing magnitude and recording on the same holographic recording medium two successive holograms of the structure while the latter is in these stress conditions. The resulting holographic recording is an interferogram which may be holographically reconstructed to produce a deformation fringe pattern whose fringe lines depict or represent the deformations occurring in the structure due to the change from one stress condition to the other. This inspection procedure is repeated periodically using the same stress conditions, and the deformation fringe patterns of the successive interferograms are compared to determine differences, if any, in the fringe patterns. Such differences, if any, between the successive fringe patterns are indicative of a change in the structural integrity or stiffness of the structure in the intervals between recording of the interferograms.
The two stress conditions required for each periodic inspection of the structure may be established by either or both static or dynamic loading of the structure. According to the static loading procedure, the structure to be inspected is subjected to a given static load, which may be simply the weight of the structure or an additional static load, during recording of the first hologram. The static load on the structure is then changed and the second hologram is recorded. According to the dynamic loading procedure, an impact or impulsive load is applied to the structure to effect propagation of stress waves through the structure. These stress waves establish a first stress in the structure when the first hologram is recorded and a second stress condition when the second hologram is recorded. According to the combined static and dynamic loading procedure, the structure is subjected to a constant static load in addition to the impulsive load.
The inspection apparatus of the copending application Ser. No. 456,998 for practicing the inspection technique of the application comprises a holographic recording unit for recording successive holograms of a selected region of the structure under inspection, an impulser for dynamically loading the structure to propagate stress waves through the region, and control means for operating the recording unit and impulser in timed relation such that each pair of successive holograms are recorded under different stress conditions in the region of interest of the structure to produce a holographic interferogram defining a deformation fringe pattern. As described above, the deformation fringe patterns of successive interferograms are compared to determine a change, if any, in the structural integrity of the structure.